Method and apparatus for photoelectric inspection of sheet materials

ABSTRACT

THIS INVENTION RELATES TO SYSTEMS FOR PHOTOELECTRIC INSPECTION OF SHEET MATERIALS WHEREIN DEFECTS SUCH AS WRINKLES, PROTUBERANCES AND STICKING SUBSTANCES LYING LENGTHWISE ON SHEETS CAN BE INFALLIBLY DETECTED BY SHOOTING A NARROW BEAM OF LIGHT WIDTHWISE, SHEERLY CLOSE TO AND ACROSS THE SURFACE OF THE MOVING SHEET MATERIAL, AND SIMULTANEOUSLY, PHOTOELECTRICALLY DETECTING ANY INFLUENCE ON THE SAID LIGHT BEAM, SUCH AS INTERCEPTION OR REFLECTION, BY DEFECTS ON THE SHEET MATERIAL.

United States Patent lnventors Hiroo Akamatsu Takatsuki; Takanobu Morita, Otokuni-gun, Japan Appl. No, 814,309 Filed Apr. 8, 1969 Patented June 28, 1971 Assignee Omron Tateisi Electronics Co.

Ukyo-ku, Kyoto, Japan Priority Apr. 8, 1968 Japan 43-28109 METHOD AND APPARATUS FOR PHOTOELECTRIC INSPECTION OF SHEET MATERIALS 9 Claims, 9 Drawing Figs.

1nt.Cl ..G0ln 21/32 Field of Search 250/219 (DF), 233, 219(RG);356/237,238, 199, 200

[56] References Cited UNlTED STATES PATENTS 2,429,331 10/1947 Sachtleben 250/219X 3,041,461 6/1962 Lindemann et a1. 356/199X 3,141,095 7/1964 Klose 250/233 3,206,606 9/1965 Burgo et al. 250/219 3,283,162 11/1966 Quittner 250/219 3,401,267 9/1968 Engle et al. 356/238X Primary ExaminerWalter Stolwein Attorney-Craig, Antonelli, Stewart and Hill ABSTRACT: This invention relates to systems for photoelectric inspection of sheet materials wherein defects such as wrinkles, protuberances and sticking substances lying lengthwise on sheets can be infallibly detected by shooting a narrow beam of light widthwise, sheerly close to and across the surface of the moving sheet material, and simultaneously, photoelectrically detecting any influence on the said light beam, such as interception or reflection, by defects on the sheet material.

PATENTED M28 law 3588 S13 SHEET 1 [1F 4 I nvenlor: mm 4mm ml mmmolll Max/r4 Pmsmmmzsmn 3,588,513

SHEET 0F 4 sssasaa DIFFZ AMP.

TRANSDUCERS A Home y METHOD AND APPARATUS FOR PHOTOELECTRIC INSPECTION OF SHEET MATERIALS This invention relates to a system for photoelectric inspection of sheet materials. This invention is applicable, for instance, to apparatus for photoelectrically detecting such defects as wrinkles, protuberances or sticking substances, etc., in the manufacture of paper, fabrics and various filmy materials, and resultantly removing defective sheets from piles of cut sheets, by means of the detecting signal.

It is an object of the present invention to provide a method and apparatus for thoroughly inspecting defects such as wrinkles, protuberances or sticking substances.

It is another object of the present invention to provide a method and apparatus for inspecting the said defects lying parallel to the direction of movement of a sheet material.

A further object of the present invention is to provide a method and apparatus for thoroughly inspecting long defects such as wrinkles, protuberances or sticking substances, respectively lying lengthwise in parallel to the direction of movement of a sheet material.

Still a further object of the present invention is to provide a method and apparatus for thoroughly inspecting long defects such as wrinkles, protuberances, or sticking substances, respectively lying lengthwise in parallel to the direction of movement of a sheet material, as well as defects such as holes and soiled spots or areas.

In this specification, the work lengthwise" is used in the sense of or in the direction of movement" of sheet materials, and the word widthwise" is used in the sense of or in the direction perpendicular to that of movement" of sheet materials.

Conventionally, defects such as wrinkles, protuberances or sticking substances on sheet materials have been inspected by, for instance, a detecting arm touching lightly on the surface of moving sheet, which is connected to a transducer for generating a detection signal. Such a conventional method has a drawback in that lengthwise defects can hardly be detected on account of the gradual movement of the detecting arm. Moreover, such mechanical detection means has a low sensitivity.

According to the method and apparatus of this invention, a detection signal of adequate amplitude can be obtained whenever a lengthwise defect is detected, thus ensuring an infallible detection.

The apparatus for photoelectric inspection of sheet materials of the present invention comprises, at least one light beam emitting means for emitting a narrow light beam which travels in the direction perpendicular to the direction of the movement of the sheet material and sheerly close to the surface thereof, at least one photoelectric transducer means for photoelectrically detecting total or partial reflection of the said light reflected at defective parts on the sheet material, and at least one amplifier means for amplifying the electric signals from the photoelectric transducer means.

The said narrow light beam is emitted in the direction perpendicular to that of movement of the sheet material, i.e., widthwise thereto, and travels sheerly close across the surface of the sheet material so as to be infallibly intercepted and reflected by the defective parts such as wrinkles, protuberances or sticking substances. The light beam thus emitted is reflected by the defective parts in such a manner that any defects intercept and reflect the light beam totally or partly depending on the degree of the defects. The said photoelectric transducer means catches such a light beam reflection and generates electric signals corresponding to the defects of the sheet material. In order to emit the narrow light beam to travel very close to the surface and widthwise fully across the sheet material, a light beam emitted from a light source composed for instance of a gas LASER and converging optical means, is preferably employed. The said photoelectric transducer means is for photoelectric detection of changes in the light beam caused by total or partial interception and reflection of the light beam by the defective parts. Accordingly, the transducer can be operated to detect total or partial reflection of the light beam by defective parts and to detect total or partial interception of the light beam by defective parts. The amplifier is for amplifying the output signal of the photoelectric transducer enabling such performances as to mark the sheet material by ink or to remove sheet material carrying defective parts from a pile of cut sheets. AC amplifiers are preferable to DC amplifiers for the reason of the more stable performance thereof.

Other objects and advantages will be obvious from the following description in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view showing the principal parts of a first embodiment of the present invention;

FIGS. 2 to 6 are perspective views respectively showing further embodiments of the present invention;

FIG. 7 and FIG. 9 are side views respectively showing still further embodiments; and

FIG. 8 is a block diagram showing a control system for use with the present invention.

In the first embodiment shown in FIG. 1, sheet material 1 is driven in a direction indicated by arrow :1 by a driving means (not shown in the FIG.) and passes over a roller 2. Accordingly, when no defects exist, the surface of the sheet material 1 passes along a path of certain cylindrical shape on the roller 2. A narrow light beam LW is emitted from a light beam emitting means 6, for instance a gas LASER light source combined with a converging optical means, to travel widthwise sheerly close to and across the surface of the sheet material at the place of its passing over the roller 2. The light beam LW, after travelling widthwise across the sheet material 1, strikes a photoelectric transducer means 7, for instance a solar battery or phototransistor, installed on the opposite side to the light beam emitting means 6 across the sheet material 1. The photoelectric transducer means 6 then generates electric signals corresponding to changes of the incoming light beam.

Above another part of the sheet material 1 there is provided a light source covering the full width of the sheet material 1 to shoot a light thereto, and below the surface of the sheet material 1 opposite to the said light source there is provided close to the sheet material 1 a photoelectric transducer means 4 consisting of juxtaposed photoelectric transducer elements, for instance solar batteries or phototransistors, 41, 42 ..4n. A lengthwise defect 8, such as a wrinkle, protuberance or a sticking substance, being short perpendicularly to the said direction of movement does not induce sufficient changes in the incoming light to the transducer elements of photoelectric transducer means 4, and therefore, sufficient electric signal cannot be generated by this detector arrangement for such a defect. In contrast, the lengthwise defect 8 imparts to the narrow light beam LW emitted widthwise to the sheet material 1 a distinct effect by totally or partly intercepting the light beam LW. Consequently, the photoelectric transducer means 7 generates a sufi'rcient electric signal corresponding to the distinct change in the incoming light when the lengthwise defeet 8 crosses the light beam LW. On the other hand, widthwise defect 5, such as a hole or a soiled spot on the sheet material 1, when it passes over the photoelectric transducer means 4, provides to the transducer elements an impulsive change in the incoming light in the form of an increase or decrease in the interception of the light LP penetrating through the sheet material 1, and results in the generating of an impulsive electric signal by the photoelectric transducer means 4. Thus, by means of the above-mentioned inspection apparatus, defects, such as holes or soiled spots, as well as lengthwise defects, such as wrinkles, protuberances or sticking substances, can be detected.

In the second embodiment shown in FIG. 2, a sheet material 1 is driven in a direction indicated by arrow a by a driving means (not shown in the FIG.) to pass over a roller 2. Accordingly, when no defects exist, the surface of the sheet material 1 passes along in a certain cylindrical path over the roller 2. A narrow light beam LW is emitted from a light beam emitting means 6, for instance a gas LASER light source combined with converging optical means, to travel widthwise sheerly close to and across the surface of the sheet material l at the place of its passing over the roller 2. Over the path of the light beam LW is provided a photoelectric transducer means 9 consisting of photoelectric transducer elements 9E, 92 ..9n in a position to cover the light path below. Consequently, when a lengthwise defect 9, such as a wrinkle, a protuberance or a sticking substance, intercepts totally or partly the light beam UN and thereby reflects the light, such reflected light strikes the nearest photoelectric transducer element of the photoelectric transducer means 9, and causes the said transducer element to generate an electric signal. When a widthwise defect 5, such as a wrinkle, a protuberance or a sticking material, crosses the path of the light beam LW, the light beam is intercepted for a short time causing a reflection of the light, and causes the transducer means 9 to generate an electric impulse.

In the embodiment shown in FIG. 3 a widthwise elongated light source 3 is provided under a sheet material ll moving in a lengthwise direction as indicated by arrow 0, and close across and above the surface of sheet material 1 is emitted a widthwise travelling narrow light beam LW provided by light beam emitting means 6, for instance a gas laser light source combined with converging optical means. Over the path of the light beam LW is provided a photoelectric transducer means 9 consisting of photoelectric transducer elements 91, 92 ..9n in a position to cover the path of the light beam LW.

A widthwise defect 5 such as a hole or soiled spot tem porarily and partly increases or more completely intercepts the light from the light source 3 penetrating through the sheet material 1 as it moves on, thereby producing a change in the incoming light to some of the photoelectric transducer elements of the photoelectric transducer means 9, and consequently, generating an electric signal. On the other hand, a lengthwise defect 9 such as a wrinkle, a protuberance or a sticking substance intercepts totally or partly the light beam LW and reflects such light as the sheet material ll moves on, thereby providing a change in the incoming light to the nearest photoelectric transducer element of the photoelectric transducer means 9, and consequently, generating an electric signal. In this particular example, either the "Penetrating light method or the Widthwise light beam method of the present invention is applicable using the common photoelectric transducer means.

In the embodiment shown in FIG. 4, a sheet material I is driven in a direction indicated by arrow a by driving means not shown in the F16. while passing over a roller 2. Accordingly, when no defects exist, the surface of the sheet material ll passes along a certain cylindrical path on the roller 2. A pair of light beam emitting means 6 and 6', for instance a pair of gas LASER light sources combined with converging optical means, is installed just off each widthwise end of the sheet material 11 in such a manner as to emit a pair of narrow light beams LW and LW which travel widthwise in opposite directions but as close as possible to each other and also sheerly close to the surface of the sheet material l at the place of its passing over the roller 2. Over the paths of the light beams LW and LW is provided a photoelectric transducer means 9 consisting of photoelectric transducer elements 91, 92, 9n. Consequently, when lengthwise defects 8 and 8, such as wrinkles, protuberances or sticking substances, respectively, intercept totally or partly the light beams UN and LW from the respective light beam emitting means 6 and 6', and thereby reflect the light therefrom, such reflected light from the beams respectively strikes the nearest photoelectric transducer elements of the photoelectric transducer means 9, and respectively causes the said transducer elements to generate electric signals.

In this embodiment two light beams LW and LW are emitted in opposite directions and in closely adjacent lines to each other from each light source provided just off each edge of the sheet material 1, and therefore, even when the surface of roller 2 is not strictly straight, the light beams LW and LW are able to travel very closely to the surface of the sheet material it on the roller 2 sufficiently far in the opposite direction that at least each half width of the sheet material 1 is subjected to precise inspection. Such an arrangement is useful in the case where the center part of the roller is made slightly thicker than either end for stability of the sheet material on the roller, or in the case where the roller is as long as several meters and therefore has a tendency to sag slightly.

In the embodiment shown in FIG. 9, a sheet material 1 is driven in a direction indicated by arrow a by driving means (not shown in the FIG.) while passing over a roller 2. Accordingly, when no defects exist, the surface of the sheet material ll passes along in a certain cylindrical path over the roller 2. A narrow light beam LWM is emitted from a light beam emitting means 6, for instance a gas LASER light source combined with converging optical means, to travel widthwise sheerly close to and across the surface of the sheet material at the place of its passing over the roller 2. The said light beam LWM is modulated by the rotary chopper plate 61 which has several holes 62 and is rotated by a motor 64 through a belt 63. Over the path of the light beam LWM is provided a photoelectric transducer means 9.

As the light beam LWM is modulated, the reflected light becomes modulated, too. Therefore, the photoelectric transducer means 9 is able to catch such modulated light even when the lengthwise defect 8 is very long, and accordingly the photoelectric transducer means 9 generates an AC electric signal which continues while the long defect 8 passes through and across the light beam LWM. Although one modulated light beam from a light beam emitting means is employed in this example, a pair of light beams respectively emitted widthwise in close parallel lines from each light beam emitting means provided just off each edge of the sheet material may be preferably employed, such as provided in the embodiment shown in FIG. 4, when the sheet material has the extraordinarily large width.

In the embodiment shown in FIG. 6, a sheet material l is driven in a direction indicated by arrow 0 by a driving means (not shown in the FIG.) while passing over a roller 2. Accordingly, when no defects exist, the surface of the sheet material 1 passes along in a certain cylindrical path over the roller 2. A pair of light beam emitting means 6 and 6, for instance a pair of gas LASER light sources combined with converging optical means, is installed just off each edge of the sheet material I in such a manner as to emit a pair of narrow light beams LWM and LW which travel widthwise in the opposite directions but as close as possible to each other and also sheerly close to the surface of the sheet material l at the place I of its passing over the roller 2.'The said light beam LWM is modulated by the rotary chopper plate 61 which has several holes 62 and is rotated by a motor 64 through a belt 63. The other light beam LW is unmodulated.

Over the path of the light beam DWM and LW is provided a photoelectric transducer means 9 consisting of photoelectric transducer elements 91, 92, ..9n. Consequently, when a lengthwise defect M such as a wrinkle, a protuberance or a sticking substance intercepts totally or partly the light beam LWM and LW and thereby reflects the light therefrom, such reflected light respectively strikesthe nearest photoelectric transducer elements of the photoelectric transducer means 9. As the light beam LWM is modulated, the reflected light becomes modulated, too. Therefore, the photoelectric transducer means 9 is able to detect such modulated light even when the lengthwise defect 81 is very long, and accordingly the photoelectric transducer means 9 generates an AC electric signal which continues while the long defect 81 passes through across the light beam LWM. On the other hand, the unmodulated light beam LW infallibly catches a short lengthwise defeet 82 such as a wrinkle, a protuberance or a sticking substance, whenever it comes to cross the path of the light beam LWM, even when the light beam LWM is chopped off by the rotary chopper plate 61, thereby causing the generation of an electric impulse by the transducer means 9. Accordingly, by amplifying the output signal of the transducer by means of an AC amplifier, the short lengthwise defects as well as long lengthwise defects can be infallibly detected.

In the embodiment shown in FIG. 7, a sheet material 1 is driven in a direction indicated by arrow a by a driving means (not shown in the FIG.) while passing over the roller 2. Accordingly, when no defects exist, the surface of the sheet material 1 passes along a certain cylindrical path over the roller 2. A narrow light beam LW is emitted from a light beam emitting means (not shown in the FIG.), for instance of gas LASER light source combined with converging optical means, to travel widthwise sheerly close to and across the surface of the sheet material 1 at the place of its passing over the roller 2. Above the path of the light beam LW is provided a photoelectric transducer means 9 consisting of closely juxtaposed photoelectric transducer elements. Over another part of the sheet material 1 is provided an elongated light source 3 which extends widthwise across the'sheet material 1 and emits a plate-shaped light flux LR onto the surface of the sheet material 1. Closely above the part of the surface of sheet material 1 illuminated by the light flux LR there is provided a photoelectric transducer means 4 consisting of closely juxtaposed photoelectric transducer elements. A lengthwise defeet 8, such as a wrinkle, a protuberance or a sticking substance, being short' perpendicular to the said direction of movement, does not induce sufficient change in the incoming light to the transducer elements of the photoelectric transducer means 4, and therefore, a sufficient electric signal for detecting purposes cannot be generated. On the other hand, the lengthwise defect 8 imparts a distinct efiect on narrow light beam LW emitted widthwise to the sheet material 1 by totally or partly intercepting the light beam LW and reflects such light as the sheet material 1 moves on, thereby providing a change in the incoming light to the nearest photoelectric transducer element of the photoelectric transducer means 9, so that a sufficient electric signal is generated. That is to say, a detection signal by the widthwise light beam method" is obtained from the photoelectric transducer means 9. A widthwise defect 5 such as a hole or soiled spot on the sheet material 1 gives the transducer elements of the photoelectric transducer means 4 an impulsive change in the incoming reflected light caused by a temporary change of a part of reflected light flux Lr when the defect passes under the photoelectric transducer means 4, and results in the generating of an electric impulse therefrom. That is to say, a detection signal by the Reflected light beam method" is obtained from the photoelectric transducer means 4.

An electrical control system for use with the invention is shown by the block diagram in FIG. 8, wherein numerals 41, 42, 43, 45, 46, 47 and 48, respectively, indicate widthwise juxtaposed photoelectric transducer elements which constitute photoelectric transducer 4. Numerals 91, 92, 93, 94, 95, 96, 97 and 98, respectively indicate widthwise juxtaposed photoelectric transducer elements which constitute photoelectric transducer means 9. The photoelectric transducer elements 41 8!. 91, 42 & 92, 43 8t 93 and so on to 48 & 98, constitute respective pairs for detecting respective regions of the sheet material 1, and are connected with each other through variable amplitude preamplifiers 101, 102, 103 and so on to 108, respectively. Alternate outputs of the said elements are connected to each pair of input terminals of differential amplifiers 111, 112, 113 and 114, respectively. The output signals of differential amplifiers 111, 112, 113 and 114 are connected to discriminators 121, 122, 123 and 124, respectively, for discriminating the said output signals and generating the detection signals. Respective output signals of the discriminators 121, 122, 123, and 124 are connected to common output terminal 130.

Accordingly, the detection signal is given when an electric signal is generated by any one of the photoelectric transducer elements 41 to 48 paired with 91 to 98, respectively, by a detection of a change in the incoming light. As the photoelectric transducer means 9 for detecting the defect by the widthwise light beam method" generates neither electric signals nor noise when there exists no defect, the photoelectric transducer means 9 has a very high value of SN ratio. On the other hand, the output signal of the photoelectric transducer means 4 is accompanied by a considerable amount of noise, and therefore, its SN ratio is not necessarily high, because the transducer means 4 is always receiving the reflected light Lr from the surface of sheet material 1. The variable amplitude preamplifiers 101 to 108 are for mixing, after adjusting, the output signals of the photoelectric transducer means 4 with the electric signals of the photoelectric transducer means 9, so that the noise from the transducer means 4 shall not be erroneously detected by the discriminators 121 to 124. Owing to the high SN ratio of the photoelectric transducer means 9, the mixing of the output signals of the transducer means 9 with those of the transducer means 4 is not liable to lower the detection ability of the transducer means 4. Accordingly, by amplifying the output signals of both transducer means 4 and 9 with the common differential amplifiers 111 to 114, it is possible to obtain from the output terminal electric signals corresponding to all kinds of defects.

In the example shown in FIG. 9, a sheet material 1 is driven in a direction indicated by arrow a by a driving means (not shown in the FIG.) while passing over rollers 2 and 2'. When no defects exist, the surface of the sheet material 1 passes along a certain cylindrical path on the roller 2. A narrow light beam LW is emitted from a light beam emitting means (not shown in the FIG.), for instance a gas LASER light source combined with converging optical means, to travel widthwise sheerly close to and across the surface of the sheet material 1 at the place of its passing over the roller 2. Over the path of the light beam LW is provided a photoelectric transducer means 9 consisting of closely juxtaposed photoelectric transducer elements. Under another part of the sheet material 1 is provided an elongated light source 3 which extends widthwise across the sheet material 1 and emits a plate-shaped light flux LP onto the back surface of the sheet material 1. Above the sheet material in the opposite position to light source 3 and close to the surface of sheet material 1 is provided a photoelectric transducer means 4 consisting of closely juxtaposed photoelectric transducer elements. A lengthwise defeet 8 such as a wrinkle, a protuberance or a sticking substance does not give sufficient change in the incoming light to the transducer elements of the photoelectric transducer means 4, and therefore, sufficient signal cannot be generated. On the other hand, lengthwise defect 8 imparts a distinct effect to the narrow light beam LW emitted widthwise to the sheet material 1, by totally or partly intercepting and reflecting light beam LW, and thereby reflects such light as the sheet material 1 moves on, thereby producing a change in the incoming light to the nearest photoelectric transducer element of the photoelectric transducer means 9, and consequently, generating an electric signal. That is to say a detection signal by the "widthwise light beam method" is obtained from the photoelectric transducer means 9.

A widthwise defect 5 such as a hole or a soiled spot on the sheet material 1 gives the transducer elements of the photoelectric transducer means 4 an impulsive change in the incoming penetrating light caused by a temporary increase in or greater interception of a part of the light LP penetrating through sheet material 1, when the defect 5 passes under the photoelectric transducer means 4, and results in the generating of an electric impulse therefrom. That is to say, a detection signal by the Penetrating light beam method is obtained from the photoelectric transducer means 4. The electrical control system for this embodiment can be arranged in the manner described with reference to FIG. 8.

As can be understood by the above mentioned description regarding various embodiments, the method and apparatus for photoelectric inspection of sheet materials according to the present invention are believed to be very useful in view of their ability of performing thorough detection of lengthwise defects such as wrinkles, protuberances, or sticking substances on sheet materials, which has hitherto been considered unfeasible by conventional methods and apparatuses.

We have shown and described several embodiments in accordance with the present invention. it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art and we, therefore, do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.

We claim:

1. An apparatus for photoelectric inspection of moving sheet material, comprising at least one light beam emitting means for emitting a narrow light beam which travels in the direction perpendicular to the direction of movement of the sheet material and sheerly close to the surface thereof; at least a first photoelectric transducer means for photoelectrically detecting total or partial reflection of the said light beam reflected at defective parts on the sheet material, said first photoelectric transducer means being provided to extend widthwise over the width of the sheet material with said light beam being interposed between the sheet material and the transducer means; and at least one amplifier means for amplifying the electric signals from the said photoelectric transducer means.

2. An apparatus for photoelectric inspection of sheet material as defined in claim I, wherein a pair of the light beam emitting means is provided for emitting adjacent lines of narrow light beams in opposite positions off each lateral edge of the sheet material.

3. An apparatus for photoelectric inspection of sheet material as defined in claim 1, wherein said at least one narrow light beam is a modulated light beam and the said amplifier means is an AC amplifier.

4. An apparatus for photoelectric inspection of sheet material as defined in claim 1, wherein there is provided a pair of light beam emitting means for respectively emitting a modulated narrow light beam and an unmodulated narrow light beam whose paths are in close proximity to each other, and the said amplifier means includes AC amplifiers.

5. An apparatus for photoelectric inspection of sheet material as defined in claim 1, further including an elongated light source extending widthwise to the sheet material in an opposite corresponding position to said first photoelectric transducer means with the sheet material interposed between the light source and the transducer means.

6. An apparatus for photoelectric inspection of sheet material as defined in claim 1, further including an elongated light source extending widthwise to the sheet material and a second photoelectric transducer means positioned opposite to said elongated light source with said sheet material interposed therebetween.

7. An apparatus for photoelectric inspection of sheet material as defined in claim 8 further including signal mixing means for mixing the signals generated by said first and second photoelectric transducer means and discriminating means for discriminating the mixed signals and generating defect signals.

3. An apparatus for photoelectric inspection of sheet material as defined in claim ll further including an elongated light source extending widthwise to the sheet material and a second photoelectric transducer means positioned on the same side of said sheet material as said elongated light source and adjacent thereto.

9. An apparatus for photoelectric inspection of sheet material as defined in claim 10 further including signal mixing means for mixing the signals generated by said first and second photoelectric transducer means and discriminating means for discriminating the mixed signals and generating defect signals. 

